Saying that "FB is not a single gender platform" is like saying that "Higher education is not a single gender platform".
You're missing their point about using limits on the market supply to discriminate for a job - i.e. even if that job is "open to all", but you've limited applicants to one gender.
HR and legal depts that don't want their company to be sued for violating Title VII will make sure that they aren't recruiting only from Barnard, unless they're some bona fide exception (e.g. hiring models for women's clothing). This idea that they need to recruit from men's only career fairs is your own.
As other commenter pointed out, how is this different from advertising in women/men magazine? Should we ban those as well? Sure, one can argue that their readers are only 98% one gender. But then some people on FB lie about their gender and some browse FB on somebody else's account. E.g. My GF never logs out off Facebook on shared devices and I see ads targeting women all the time.
All in all, we should look at bigger problem of targeted advertising rather exclusive loaded cases. I've no idea what targeted advertising line is "good enough". On the other hand, targeted advertising is definitely causing massive societal problems and political polarisation.
Facebook is not a single-gender platform.